This invention concerns ion-plasma treatment of materials and, more particularly filtered high quality cathodic-arc plasma sources usable for such treatment.
Vacuum-arc methods of coating deposition and surface modification have gained wide recognition in recent decades for use in for example tool production, mechanical engineering and instrument making. These processes form desirable coatings and surface layers that cannot be produced by other known methods. The essence of and the areas of application of vacuum-arc coating formation and surface modification have been described for example in the technical journal articles “Principles and Applications of Vacuum Arc Coatings” by R. L. Boxman, IEEE Transactions on Plasma Science, Volume 17, Number 5, October 1980 and in “Metal Plasma Immersion Ion Implantation and Deposition: a Review” by A. Anders, Surface and Coating Technology, 93 (1997) pages 158-167.
The presence of macroparticles of cathode material in cathodic-arc plasma often deteriorates the quality of a synthesized coating and a surface treated in this manner. This deterioration in fact hinders an even wider application of the vacuum arc technology. This deterioration especially affects such demanding applications as microelectronics, optics, fine mechanics, and medicine (e.g. surgical instrument sets and prosthetics). Problems associated with macroparticle generation and transport, and also methods of suppressing macroparticle flow between the cathode of an arc source and the treated surface have been considered in the published article “Macroparticle Contamination in Cathodic Arc Coating: Generation, Transport and Control” by R. L. Boxman and S. Goldsmith, Surface and Coating Technology, 52 (1992) p. 39-50. The Boxman et al. and each of the other references, including patent documents, identified in this disclosure are hereby incorporated by reference herein.
Magnetic filters currently provide the most efficient cleaning of the plasma generated in vacuum-arc cathode spots. The action of these filters is based on spatial separation of the trajectories of plasma components i.e., separation of ions and macroparticles. Between the substrate or work piece (the object to be treated) and the active cathode surface, which emits plasma flow inclusive of macroparticles, an obstacle is installed to exclude the direct line-of-sight between the cathode and the substrate. A special screen or the walls of a bent tubular plasma duct can serve as this obstacle. In such systems, ions are bypassed around the obstacle to the output of the system, and further, to the substrate using electromagnetic field guidance. Heavy and weakly charged macroparticles and neutral atoms are however intercepted by the screen or plasma duct walls, since they are less responsive to magnetic and electric fields and tend to move in rectilinear trajectories.
A known cathodic-arc plasma source comprises a cylindrical or conical cathode with a working end surface evaporated by the cathode spot of an arc, a tubular cylindrical anode and a cathode coil that surrounds the cathode and generates a magnetic field confining the cathode spot on the working end of the cathode. This source also includes an anode magnetic coil serving to magnetically focus the plasma stream emitted by the cathode spot. An apparatus of this type is shown in the I. I. Axenov (Aksenov) et al., U.S. Pat. No. 4,551,221 of 1985. In this source, substantially the whole ion component is directed by the focusing magnetic field to the source output, and the macroparticles, which move mainly in the radial direction, are intercepted by the anode walls, so that the macroparticle concentration in the output plasma flow is substantially reduced with respect to plasma sources having no focusing magnetic coil.
A more effective suppression of macroparticle flow is ensured when a screen is placed on the axis of the tubular anode or the plasma duct to intercept the part of macroparticle flow moving along the device to its output; this arrangement is shown in the I. I. Axenov (Aksenov) et al., U.S. Pat. No. 4,452,686 of 1984. The output of the useful (ion) plasma component is however moderate in this device because of great ion losses on the incorporated screen.
Cathodic-arc filtered-plasma sources, wherein macroparticles are removed from an erosion plasma as it travels along a curvilinear plasma duct have gained the widest recognition in production practice, see I.I. Aksenov et al., “Transport of Plasma Stream in a Curvilinear Plasma-Optics System”, Soviet Journal of Plasma Physics 4 (4), 1978, p 425-428. In these devices, charged particles, i.e., electrons and ions, are transported along the plasma duct by the magnetic field of magnetic coils arranged evenly over the plasma duct length. In contrast the heavy and weakly charged macroparticles, being unresponsive to magnetic and electric fields, and moving by inertia in straight-line trajectories, inevitably encounter the plasma guide channel walls. After collision with a channel wall however a considerable number of macroparticles do not stick to the wall and may maintain some kinetic energy even after a few collisions. As a result, an appreciable part of the rebound macroparticles may arrive at the exit port of the plasma duct and thus at the work piece or substrate. The number of rebounding macroparticles at the output of the plasma duct is significantly reduced when intercepting screens (e.g., a set of plane fins) are arranged on the plasma duct walls to serve as traps of macroparticles.
The efficiency of macroparticle removal with a curvilinear magnetic filter can be improved by lengthening the plasma-guiding channel, or by decreasing its width and/or increasing the total bend angle of the channel [see Xu Shi et al., “Filtered Cathodic Arc Source”, International Patent Application No. PCT/GB/00389, Int. Pub. No. WO 96/26531; S. Anders et al., “S-shaped Magnetic Macroparticle Filter for Cathodic Arc Deposition”, Proc. XVIIth International Symposium on Discharges and Electrical Insulation in Vacuum, Berkeley, Calif., Jul. 21-26, 1996, p.904]. In these systems however, the losses of the ion component are increased and the throughput of the plasma stream is significantly decreased. The productivity of such systems is reduced, while design complexity and, correspondingly, costs are increased. As a result, the applicability of these systems in the coating production practice is rather limited.
Other known plasma arrangements attempt to improve the efficiency (maximize throughput of the plasma stream and minimize macroparticle transport) of the curvilinear plasma filters and simplify their design. For example, V. I. Gorokhovsky in U.S. Pat. No. 5,435,900, 1995, teaches minimizing the length of the plasma guiding channel with a relatively great cross-sectional area, while S. Falabella et al., in U.S. Pat. No. 5,279,723, 1994 teaches use of a simple magnetic system in the form of two magnetic coils embracing two straight tubular plasma ducts, that are joined at 45° to each other. These simplifications in filter design are not however accompanied by an adequate improvement of the filter efficiency. This is explained by the fact that such simplification of the magnetic filter system deteriorates the transport properties of the magnetic field itself. This deterioration is especially notable where regions with high longitudinal and transverse field gradients appear. Such gradients act as magnetic “mirrors” and hinder plasma passage along the system. Another negative consequence of these simplifications is a degradation of the particle filtering efficiency.
Another known cathodic-arc plasma source comprises a cylindrical cathode with an end working surface; a tubular anode coaxial with the cathode; a plasma duct, electrically insulated from the anode and forming together with the anode a curvilinear plasma-guiding channel bent at 45°. This source includes electromagnetic coils embracing the cathode, the anode and the plasma duct and arranged along the whole plasma-guiding duct; see the P. J. Martin et al., U.S. Pat. No. 5,433,836, 1995. Such a moderate bend angle with magnetic field uniformly distributed along the channel provides a fair efficiency of the ion component. passage through the filtering channel i.e., the output of the ion flow is up to 2.5 Amperes measured at the filter exit with a cathode arc current of 100 Amperes. A further improvement in filter efficiency is hindered by ion losses on the duct walls due to the plasma flow displacement as a result of centrifugal and gradient drifts. Such drifts always accompany plasma passage along a curvilinear inhomogeneous magnetic field. Thus, the major part of the plasma losses occur in the curvilinear part of the plasma-guiding duct by way of the ion component drifting to the duct walls.
There are filter arrangements providing a partial compensation of the drifts during the plasma passage along the filtering plasma-guiding ducts. For example, Kim et al., U.S. Pat. No. 6,026,763 teaches the placement of additional electromagnetic coils on the convex side of the bent part of the plasma duct to repel drifted ions from the duct wall. Additionally, Gorokhovsky, U.S. Patent Application Publication No. U.S. 2002/0007796A1, 2002 teaches the placement of additional plate-shaped electrodes inside the plasma duct and use of a positive bias voltage applied to the electrodes from an individual power source to counteract ion drift. These measures however, do not provide desirable filter efficiency. Furthermore, they considerably complicate the filtering system, eliminating previous attempts toward filter simplification.
In addition none of the known filtered cathodic-arc plasma sources can provide a plasma stream of complex, i.e., multi component, composition, nor can these sources provide a uniform distribution of component concentrations in the plasma cross section, when mixing plasma streams emitted simultaneously by two and more vacuum arc plasma sources. In the known devices [i.e., U.S. Patent Application Publication No. U.S. 2002/0007796A1, 2002], the plasma generated by two sources with different evaporated cathode materials is transported in one plasma duct, but using two parallel paths displaced with respect to each other in space. As a result, the distribution of cathode material concentration in the cross section of the total plasma stream and, therefore, over the substrate surface located at the filter exit is not uniform.
The present invention is believed to provide assistance with respect to several of the difficulties identified in these discussions of known filtered cathodic-arc plasma sources. The achievement of improved plasma filter input to output current efficiencies is a particular aspect of the invention.